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Feature #1824

don't try to deactivate the SACCH in case of deactivation of PDCH channels

Added by laforge about 1 year ago. Updated 10 months ago.

Status:
Closed
Priority:
Normal
Assignee:
Category:
libbsc
Start date:
10/15/2016
Due date:
% Done:

0%

Resolution:
Spec Reference:

Description

When attempting to manually enable a PDCH on a RBS2000 without a PCU connected to the CCU, the RBS2000 will inform us using a RSL CONNECTION FAIL message. We respond by RF CHANNEL RELEASE,but we also send a DEACTIVATE SACCH from our channel deactivation logic.

As the PDCH has no SACCH (only dedicated circuit-switched channels do), this is of course bogus.

So what needs to be done is probably to check if this channel is currently activated as PDCH, and if it is, skip the deactivate SACCH step.

I believe this is not only valid for RBS2000 but also for osmocom-style dynamic PDCH support. Neels: Any thoughts on this?

rsl_chan_act_pdch.pcap (318 Bytes) laforge, 10/15/2016 09:32 PM

History

#1 Updated by laforge about 1 year ago

attaching pcap file with example.

don't be surprised if wireshark doesn't fully decode the messages. The non-standard RSL extensions require a patch (see my commit 89c71c635f0e85a66de3a176b0a0df2970d8058b in wireshark.git on osmocom.org, branch laforge/om2000-201610). However, a full decode of the RSL CHAN ACT message is not required to solve this bug.

#2 Updated by neels about 1 year ago

With my default wireshark I basically see only "malformed packet", so I guess I'd need the extensions.

I'm not familiar with SACCH, but if you say that SACCH only exists for CS channels, then the
obvious answer is that we can't deact SACCH on a PDCH channel.

The conclusion would be that rsl_rx_conn_fail() calling rsl_rf_chan_release_err() in abis_rsl.c
must not pass SACCH_DEACTIVATE unconditionally. This patch could be all that's needed:

diff --git a/openbsc/src/libbsc/abis_rsl.c b/openbsc/src/libbsc/abis_rsl.c
index b9132a5..eb33b3a 100644
--- a/openbsc/src/libbsc/abis_rsl.c
+++ b/openbsc/src/libbsc/abis_rsl.c
@@ -859,7 +859,9 @@ static int rsl_rf_chan_release_err(struct gsm_lchan *lchan)
 {
        if (lchan->state != LCHAN_S_ACTIVE)
                return 0;
-       return rsl_rf_chan_release(lchan, 1, SACCH_DEACTIVATE);
+       return rsl_rf_chan_release(lchan, 1,
+                                  ts_is_pdch(lchan->ts) ?
+                                  SACCH_NONE : SACCH_DEACTIVATE);
 }

(haven't tested nor compiled it yet)

BTW...

From 08.58 8.4.4:

CONNECTION FAILURE INDICATION
This message is sent from BTS to BSC to indicate that an active connection has been broken for some reason.

That sounds like a channel was already fully established and then fails.
From a failure to switch on a PDCH lchan, I would expect an 'Channel Activation Nack' instead?
Anyway, if the RBS2000 replies with a conn failure, we have to be able to deal with it...

#3 Updated by dexter 10 months ago

  • Status changed from New to Resolved

#4 Updated by laforge 10 months ago

  • Status changed from Resolved to Closed

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